The present invention relates to a method for correction of an actual value of a signal affected by harmonics, the actual value determined by means of a first integrating actual value acquisition device, which value is affected by errors, and the signal is generated from a harmonic-free setting signal by means of a modulation function which approximates a harmonic-free setting signal, and a control system.
In electrical drive technology, analog measurement variables, for example converter current, machine voltage, etc., are frequently converted into digital measurement values using integrating measurement value acquisition devices. With the use of an integrating measurement value acquisition device, measurement noise is filtered out and this measurement device can be structured in a very cost-effective manner. A typical structure of an integrating measurement value acquisition device is explained in greater detail in reference to FIG. 3.
Harmonics of a measurement variable are only filtered out, in this connection, if they have the mean value of zero during the measurement interval (gate time). This is brought about, for example, in space vector modulation, by approximation, in that the gate time is made exactly as large as the duration of a switching state sequence, which approximates a reference vector.
If the harmonics of the measurement variable cannot be ignored during the measurement interval (gate time), the actual value determined by means of the integrating measurement value acquisition device is affected by errors. This error is dependent on the mean value of the harmonics from one gate time to the next. The harmonics that impact the measurement variable, and the amplitude at which they impact it, are dependent on the device which excites these harmonics. In a pulse-width-modulated converter with an intermediate voltage circuit, the converter voltage can be controlled by means of precalculated, optimized pulse patterns. This converter voltage, which is affected by harmonics, is passed to an asynchronous machine. The harmonics are excited by the precalculated, optimized pulse patterns. If the torque is now supposed to be regulated in this asynchronous machine, the converter output current has to be acquired. However, since this is affected by harmonics, the actual value of the torque-producing current becomes incorrect by means of the integrating measurement value acquisition device, causing the torque regulation of the asynchronous machine to become incorrect.